1. Field of the Invention
The invention pertains to radio signal mixing apparatus and more specifically concerns the utilization of dual diodes for the achievement of high conversion efficiency signal mixers.
2. Description of the Prior Art
Frequency mixers employing diodes as the non-linear element find wide applications in microwave receivers because of their simplicity, low weight and low cost. However, these mixers exhibit signal losses in the conversion process which contribute to the noise figure of the receiver, thus limiting the range of the over-all system. Thus, a reduction of the mixer conversion loss is translated into an increase in the over-all system range or directly into a reduction of the required transmitted power.
Conventional diode mixers provide an output signal at a frequency which is the difference between the frequency of the local oscillator signal and the frequency of the received signal. Unfortunately, when these two signals are applied to the mixer diode, a great many other signals are generated. The frequency of these signals will be of the form nF.sub.LO .+-. mF.sub.S where n and m are integers, F.sub.LO and F.sub.S are the local oscillator and signal frequencies, respectively. The signal power generated at frequencies other than the difference frequency with n and m both equal to 1, are the major contributors to the conversion loss of the mixer, and the prevention of the generation of these signals can provide a significant improvement in the over-all conversion efficiency. Prior art methods of achieving this result have employed resonant circuits which suppress the unwanted mix products. However, these circuits are band limited and must be retuned whenever a diode is replaced.